1. Field of the Invention
The present invention relates to a radio analog sensor which sends, by radio, analog detection signals about a temperature, a smoke density or the like to a remote place. Particularly, the present invention relates to a radio analog sensor in which a threshold value is set on the sensor side, and analog detection signals are sent to the side of a main device when the detected value exceeds a threshold, whereby data can be sent without decreasing the lifetime of a battery.
2. Description of the Related Art
An analog fire alarm system has recently been put into practical use more and more in which analog signals about a temperature, a smoke density or the like, which is detected by an analog sensor, are sent directly to a receiver whlch decides from the received analog signals wether or not a fire starts. In such a fire alarm system, a fire decision software is loaded on the receiver so as to sample the detection output from the analog sensor at predetermined intervals, predict changes of the fire data in the future from the sampled data, and decide that a fire starts when the predicted data satisfies predetermined fire conditions. This facilitates the detection of a fire in an early stage and the prevention of false alarm.
A fire sensor using the above fire decision software is disclosed in Japanese Patent Laid-Open No. 61-233897 filed by the applicant of this invention. Namely, a decision on a fire is made on the basis of the data obtained by the predicting operation using functional approximation. The detection data of the analog sensor is first sampled by a sampling circuit and averaged by a moving average method. A check is then made as to whether or not the newest data obtained by averaging calculation exceeds an operation starting level.
The threshold levels used for decision on a fire include the operation staring level set to a predetermined level greater than stationary changes of the analog data, and the critical level set for making a decision on a fire from the predicted data. If the averaged data exceeds the operation starting level, non-fire protection processing is started. In this non-fire protect processing, for example, when twenty items of data LD1 to LD20 are successively stored in a memory by averaging calculation, if the newest data LD20 exceeds the operation staring level, changes in the four items of data LD17 to LD20, i.e., the slopes Y1, Y2 and Y3, are detected. A check is then made as to whether or not the positive slopes Y2 and Y3 are greater than a specified slope value Yk, and the number N of slopes greater than Yk is counted. If the count number N is 2 or more, it is decided that there is the danger of a fire, and the predicting operation is started by the functional approximation. For example, when there are two slopes Y2 and Y3 greater than the specified slope value Yk, since the data exceeds the operation staring level with the above slope changes, the predicting operation is performed. On the other hand, when there is only one slope greater than Yk, it is decided that the data change is caused by cigarette smoke or the like, the predicting operation by the functional approximation is not performed.
When the data passed through the non-fire protection processing is obtained, the operation of predicting data changes in the future is performed by a quadratic functional approximation. The principle of the predicting operation by the functional approximation is as follows:
A change of a temperature or smoke density with time in a fire is approximated to the following equation: EQU Y=ax.sup.2 +bx+c
The coefficients a, b and c of the quadratic functional equation are determined by the method of least squares using the twenty items of data LD1 to LD20 which have been obtained at the same time as the start of operation. If the coefficients a, b and c can be calculated by the above method, data changes in the future can be predicted.
A predicted time Tpu of arrival to the critical level is then calculated. The predicted critical level arrival time Tpu can be determined by determining the time tr of arrival of the locus of the data changes in the future, which is presented by the quadratic function obtained by the predicting operation, to the critical level, and subtracting the present time tn from the critical level arrival time tr.
A check is then made as to whether or not the predicted critical level arrival time Tpu is smaller than a predetermined critical time, e.g., 800 seconds. The shorter the predicted critical level arrival time Tpu, the higher the possibility of a fire. When the predicted time Tpu is 800 seconds or less, it is thus decided that a fire starts, and a fire signal is output.
However, such an analog fire alarm system employs a wire method in which a sensor is connected to a signal line from the receiver, as in a conventional fire alarm system. The analog fire alarm system thus has no merit from the viewpoints of the labor for wiring between the receiver and the sensor and the cost.
A radio alarm system has been thus proposed, which has the greatest merit that it can make wiring between the receiver side and the sensor side unnecessary and which is mainly used in a site of construction and the like.
In a current radio alarm system, when a fire is detected by an on-off type fire sensor, a fire detection signal is sent to the main device side by radio, and a fire alarm is displayed. However, as is obvious from the flow of a wire fire alarm system, the need for a radio analog sensor which performs the above-described analog data processing will be certainly produced in the near future.
A conventional system similar to a radio analog sensor of the above type is a data transmitter such as a telemeter or the like.
A telemeter system employs a method of ordinarily transmitting radio waves or transmitting radio waves by polling from a main station in order to send data about the flow of a river, weather conditions or the like. The telemeter system also generally uses a commercial power source.
However, when the radio transmission method in a conventional telemeter system is applied to a radio analog sensor, there are the following problems:
Although transmission in the telemeter system is long-range transmission with a transmission range of as long as several tens km, the transmission range in a fire sensor is generally as short as 1 km or less. The fire sensor thus consumes only little power for transmission and uses a battery power source for obtaining a merit by completely removing wiring.
When a battery power source is used in a fire sensor, the use of the method of a conventional telemeter system in which radio waves are ordinarily transmitted or transmitted by polling from a main device at predetermined intervals has the advantage that data can be obtained on real time, but it has the problem that the life time of the battery is significantly decreased. This causes the need for frequent change of the battery and makes the maintenance and control troublesome. The fire sensor system cannot be thus put into practical use if no improvement is made.